Exception Handling Using Try Catch in SQL Server

Exception Handling Using Try Catch in SQL Server

In this article, I am going to discuss Exception Handling Using Try Catch in SQL Server with examples. Please read our previous article where we discussed how to raise errors explicitly in SQL Server using Raise Error and Throw statement. As part of this article, we are going to discuss the following pointers.

  1. How to use Try Catch in SQL Server to Handle Error?
  2. Example To understand the Try-Catch implementation in SQL Server.
  3. Try-catch Implementation in SQL Server with system defined error statements.
  4. What is ERROR_MESSAGE() in SQL Server?
  5. A real-time example of Exception handling using the try-catch in SQL Server.
How to use Try Catch in SQL Server to Handle Error?

From SQL Server 2005 we are provided with a structure error handling mechanism with the help of TRY and CATCH blocks. The syntax of using TRY- CATCH is shown in the below image. 

Exception Handling Using Try Catch in SQL Server

The SQL statements which can have the possibility to throw an exception need to be placed in between the BEGIN TRY and END TRY blocks. If there is an exception occurred in the TRY block, then the control immediately moves to the Corresponding CATCH block. If there is no exception occurred in the TRY block, then the CATCH block simply skipped, and the statements which are presents after the CATCH block are going to be executed. 

Note: The Errors trapped by a CATCH block are not going to be returned to the calling application. If you want to return the error information back to the calling application then you need to use the RAISERROR() function explicitly with the catch block. In our previous article, we discussed how to raise error explicitly using the RAISERROR() function.

Example: To understand the Try-Catch implementation in SQL Server.

In the following example, we create a stored procedure for dividing 2 variables values by using the TRY CATCH implementation with user-defined error statements.

CREATE PROCEDURE spDivideTwoNumbers
@Number1 INT, 
@Number2 INT
AS
BEGIN
  DECLARE @Result INT
  SET @Result = 0
  BEGIN TRY
    SET @Result = @Number1 / @Number2
    PRINT 'RESULT IS : '+CAST(@Result AS VARCHAR)
  END TRY
  BEGIN CATCH
    PRINT 'SECOND NUMBER SHOULD NOT BE ZERO'
  END CATCH
END

Execution: EXEC spDivideTwoNumbers 10, 2

OUTPUT: RESULT IS : 5

EXEC spDivideTwoNumbers 10, 0

OUTPUT: SECOND NUMBER SHOULD NOT BE ZERO

When we execute the above-stored procedure with correct values, then the error will not occur in the program. That means after executing all the statements in the try block the control directly jumps to the statements present after the catch block without executing the catch block.

If any error occurs in the execution process i.e. in the try block, then in such case from the line where the error got occurred, the control directly jumps to the catch block. So rest of the statement in the try block will not execute whereas catch block will execute.

Note: In the above program when the error got occurred, we are displaying an error message “SECOND NUMBER SHOULD NOT BE ZERO”. In place of that error message we can also display the original error message by calling a function “Error_Message”. To test this rewriting the code inside the catch block as following

Print Error_Message()

Example: try-catch with system defined error statements:

In the following example, we create a stored procedure for dividing two variables values by using try-catch implementation with system defined error statements.

ALTER PROCEDURE spDivideTwoNumbers
@Number1 INT, 
@Number2 INT
AS
BEGIN
  DECLARE @Result INT
  SET @Result = 0
  BEGIN TRY
    SET @Result = @Number1 / @Number2
    PRINT 'RESULT IS : '+CAST(@Result AS VARCHAR)
  END TRY
  BEGIN CATCH
    PRINT ERROR_MESSAGE()
  END CATCH
END

Execution: EXEC spDivideTwoNumbers 10, 0

OUTPUT: Divide by zero error encountered.

What is ERROR_MESSAGE() in SQL Server?

This method is used to display what type of error has occurred in the try block.

A real-time example of using the try-catch implementation in SQL Server.

Please use below SQL Script to create and populate the Product and ProductSales table with some test data.

-- Create Product table
CREATE TABLE Product
(
  ProductId INT PRIMARY KEY,
  Name VARCHAR(50),
  Price INT,
  QuantityAvailable INT
)
GO

-- Populate the Product Table with some test data
INSERT INTO Product VALUES(101, 'Laptop', 1234, 100)
INSERT INTO Product VALUES(102, 'Desktop', 3456, 50)
INSERT INTO Product VALUES(103, 'Tablet', 5678, 35)
INSERT INTO Product VALUES(104, 'Mobile', 7890, 25)
GO

-- Create ProductSales table
CREATE TABLE ProductSales
(
  ProductSalesId INT PRIMARY KEY,
  ProductId INT,
  QuantitySold INT
) 
GO

SELECT * FROM Product will give us the below output

Exception Handling Using Try Catch in SQL Server

Create the following stored procedure for product sales.

CREATE PROCEDURE spSellProduct
@ProductID INT,
@QuantityToSell INT
AS
BEGIN
  -- First we need to Check the stock available for the product we want to sell
  DECLARE @StockAvailable INT

  SELECT @StockAvailable = QuantityAvailable 
  FROM Product 
  WHERE ProductId = @ProductId

  -- We need to throw an error to the calling application 
  -- if the stock is less than the quantity we want to sell
  IF(@StockAvailable< @QuantityToSell)
  BEGIN
    Raiserror('Enough Stock is not available',16,1)
  END
  -- If enough stock is available
  ELSE
  BEGIN
    BEGIN TRY
      -- We need to start the transaction
      BEGIN TRANSACTION

      -- First we need to reduce the quantity available
      UPDATE	Product SET 
          QuantityAvailable = (QuantityAvailable - @QuantityToSell)
      WHERE	ProductID = @ProductID

      -- Calculate MAX ProductSalesId
      DECLARE @MaxProductSalesId INT
      SELECT	@MaxProductSalesId = CASE 
          WHEN  MAX(ProductSalesId) IS NULL THEN 0 
          ELSE MAX(ProductSalesId) 
          END 
      FROM	ProductSales

      -- Increment @MaxProductSalesId by 1, so we don't get a primary key violation
      Set @MaxProductSalesId = @MaxProductSalesId + 1

      -- We need to insert the quantity sold into the ProductSales table
      INSERT INTO ProductSales(ProductSalesId, ProductId, QuantitySold)
      VALUES(@MaxProductSalesId, @ProductId, @QuantityToSell)

      COMMIT TRANSACTION
    END TRY
    BEGIN CATCH
      ROLLBACK TRANSACTION
      SELECT	ERROR_NUMBER() as ErrorNumber,
          ERROR_MESSAGE() as ErrorMessage,
          ERROR_PROCEDURE() as ErrorProcedure,
          ERROR_STATE() as ErrorState,
          ERROR_SEVERITY() as ErrorSeverity,
          ERROR_LINE() as ErrorLine
    END CATCH
  End
END

In the procedure spSellProduct the Begin Transaction and Commit Transaction statements are wrapped between the Begin Try and End Try block. If there is no error occurred in the code that is enclosed within the TRY block, then the COMMIT TRANSACTION statement gets executed and the changes are made permanent.

On the other hand, if there is an error within the try block then the control immediately jumps to the CATCH block and in the CATCH block, we are rolling back the transaction. So it’s much easier to handle errors with Try/Catch construct than with @@Error system function.

SQL Server also provides some built-in function that we can use in the scope of a CATCH block which is used to retrieve more information about the error that occurred and these functions will return NULL if they are executed outside the scope of the CATCH block.

Note: We cannot use the TRY/CATCH implementation within a user-defined function

In the next article, I am going to discuss the Transaction Control Language in SQL Server with some examples. Here, In this article, I try to explain Exception Handling Using Try Catch in SQL Server step by step with some examples. 

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